Here's how to fix the stupid Ranked Duel in this game:
Players should have go into match with 2 decks. And, after the opponent matching, players should go to menu where we can see who our opponent, and pick deck (out of those 2) to play.
Example: if I saw my opponent use Yami Yugi, it means I'm prolly either against Lunalight or Gaia, so I can choose which of my prepared, say Shiranui, deck that better against them - the backrow one or the turbo synchro one.
I think Ranked would be more fair that way, and push deck building better.
You are basically describing a sideboard. Only problem is, it has never been added in Duel Links and like someone has said, it'd change Ranked from a single duel format to a match format (the best out of 3 duels), making the games last much longer.
Dkayed and his lackeys already use sideboards in their own particular tournaments, but that's because they do what they please.
Why would it be longer? The duel still single one on one, using 1 deck. The suggestion just adds two more clicks before the duel begin, and the rest is the same.